What you're probably noticing
Maybe the water takes longer to get hot than it did a couple of years ago. Maybe there's a faint rumbling or popping sound coming from the tank in the garage or utility closet, something that sounds almost like a pot of water starting to boil. Maybe the hot water runs out faster than it used to, even though nobody in the house changed their shower habits. None of that means the tank is simply old. In most homes around Nixa, it means the tank is fighting the water itself.
Nixa's water is the real culprit
This part of southwest Missouri sits on a lot of limestone, and water that moves through limestone picks up calcium and magnesium on the way. That's hard water, and Nixa, along with most of Christian County, has it in good supply. City water here tests noticeably harder than water in places without that limestone base, and well water in the surrounding rural areas is often harder still.
Every time that water heats up in the tank, some of those dissolved minerals drop out as solid scale. It settles on the heating element or burner and builds up along the bottom of the tank. Over years, that layer gets thick enough to insulate the water from the heat source, which is why the unit has to work harder and longer to do the same job it used to do in a fraction of the time.
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Why this hits harder here than elsewhere
A lot of the housing stock around Nixa dates from the growth years of the 1990s through the 2010s, with a good number of newer builds added in since as the town has expanded. Many of these homes came with standard 40 to 50 gallon tank heaters tucked into a closet or garage, not the tankless systems you'd find more often in newer construction elsewhere. Tank units have more surface area for scale to collect on, and garages here swing from summer heat into winter cold, which stresses metal and fittings that are already working harder than they should.
Summers in the Ozarks get hot and humid, which means the water heater is often running flat out just to keep up with laundry, dishwashers, and extra showers. Winters bring real cold snaps, sometimes hard freezes, and incoming water temperature drops, so the unit has to heat from a colder starting point. A tank already losing efficiency to scale buildup has less margin to handle either extreme.
What you can check yourself
Look at the temperature and pressure relief valve and make sure nothing is leaking around it. Check the area under and around the tank for any dampness, rust streaks, or mineral crust, especially near the bottom seam. Listen for that rumbling sound when the burner or element kicks on. That sound is scale breaking loose and shifting around in the bottom of the tank as it heats, not air in the pipes.
You can also flush the tank yourself if you're comfortable with it. Shut off the power or gas, connect a hose to the drain valve, and run the water out into a bucket or outside until it runs clear. If what comes out looks like sand or gravel instead of water, that's your scale buildup, and it tells you plainly what's been happening inside the tank for years.
Where it stops being a DIY job
If the drain valve won't open, or opens and then won't fully close again, stop there. A valve seized by mineral deposits can strip or crack, and then you've got a tank that won't shut off. If you're getting a rumbling sound that's new and the unit is more than six or seven years old, that's worth having someone look at before it becomes a leak rather than after.
Flushing a tank regularly slows the mineral buildup but doesn't stop it, since Nixa's water supply isn't changing. A water softener on the whole house helps more than anything you can do at the tank itself, but that's a separate project and a separate decision. Either way, if the tank is leaking from the bottom, if the pressure relief valve won't reseat, or if the heater is cycling constantly without keeping up, that's the point to call in a repair technician rather than keep working on it yourself. A tank that fails from scale buildup usually gives warning signs for weeks or months before it actually leaks, so catching it early is almost always cheaper and simpler than waiting for it to fail outright.